Reducing voltage non-linearity in a bridge having tunneling magnetoresistance (TMR) elements

ABSTRACT

In one aspect, a magnetic field sensor includes a plurality of tunneling magnetoresistance (TMR) elements that includes a first TMR element, a second TMR element, a third TMR element and a fourth TMR element. The first and second TMR elements are connected to a voltage source and the third and fourth TMR elements are connected to ground. Each TMR element has a pillar count of more than one pillar and the pillar count is selected to reduce the angle error below 1.0°.

BACKGROUND

Magnetic field sensors are used in a variety of applications, including, but not limited to, an angle sensor that senses an angle of a direction of a magnetic field, a current sensor that senses a magnetic field generated by a current carried by a current-carrying conductor, a magnetic switch that senses the proximity of a ferromagnetic object, a rotation detector that senses passing ferromagnetic articles, for example, magnetic domains of a ring magnet or a ferromagnetic target (e.g., gear teeth) where the magnetic field sensor is used in combination with a back-biased or other magnet, and a magnetic field sensor that senses a magnetic field density of a magnetic field.

Various parameters characterize the performance of magnetic field sensors and magnetic field sensing elements. With regard to magnetic field sensing elements, the parameters include sensitivity, which is the change in the output signal of a magnetic field sensing element in response to a magnetic field, and linearity, which is the degree to which the output signal of a magnetic field sensor varies linearly (i.e. in direct proportion) to the magnetic field.

Some magnetic field sensors include magnetoresistance (MR) elements, such as giant magnetoresistance (GMR) elements and tunneling magnetoresistance (TMR) elements. Generally, GMR and TMR elements have a relatively high sensitivity compared, for example, to Hall effect elements. GMR and TMR elements have moderately good linearity, but over a restricted range of magnetic fields. However, even in the restricted range of magnetic fields, the linearity of the GMR or TMR element can suffer from irregularities, such as due to high temperatures.

Further, angle sensors using GMR and TMR elements can experience angle error due to phenomena such as spin flop (i.e. alteration of reference direction at high magnetic fields or anisotropy field (e.g., magnetocrystalline anisotropy or shape anisotropy)). For example, the angle obtained with GMR elements can experience error up to several degrees.

SUMMARY

In one aspect, a magnetic field sensor includes a plurality of tunneling magnetoresistance (TMR) elements that includes a first TMR element, a second TMR element, a third TMR element and a fourth TMR element. The first and second TMR elements are connected to a voltage source and the third and fourth TMR elements are connected to ground. Each TMR element has a pillar count of more than one pillar and the pillar count is selected to reduce the angle error below 1.0°.

In another aspect, a magnetic field sensor includes a plurality of tunneling magnetoresistance (TMR) elements that include a first TMR element, a second TMR element, a third TMR element and a fourth TMR element arranged in a bridge. A voltage across each TMR element is constant. The first and second TMR elements are connected to a voltage source. The magnetic field sensor also includes a plurality of transistors that include a first transistor, a second transistor, a third transistor and a fourth transistor and each transistor is connected to another transistor and to a TMR element. The magnetic field sensor also includes a first amplifier and a second amplifier. An output of the first amplifier is connected to a base of the first transistor and the output of the first amplifier is connected to a base of the second transistor. A first input of the first amplifier is connected to the first TMR element and the second input of the first amplifier is connected to the second TMR element. An output of the second amplifier is connected to a base of a third transistor and the output of the second amplifier is connected to a base of a fourth transistor. A first input of the second amplifier is connected to the third TMR element and the second input of the second amplifier is connected to the fourth TMR element.

In a further aspect, a magnetic field sensor includes a plurality of tunneling magnetoresistance (TMR) elements that include a first TMR element, a second TMR element, a third TMR element and a fourth TMR element arranged in a bridge. A voltage across each TMR element is constant. The first and second TMR elements are connected to a constant voltage source and the third and fourth TMR elements are connected to ground. The first and third TMR elements are connected to a first bridge node and the second and third TMR elements are connected to a second bridge node. The magnetic field sensor also includes a first resistor connected to the first bridge node and a first output node, a second resistor connected to the second bridge node and a second output node, a first amplifier and a second amplifier. An output of the first amplifier is connected to the first output node and the first resistor and a first input of the first amplifier is connected to the first bridge node and the first resistor. An output of the second amplifier is connected to the second output node and the second resistor. A first input of the second amplifier is connected to the second bridge node and the second resistor and a second input of the second amplifier is connected to a second input of the first amplifier.

DESCRIPTION OF THE DRAWINGS

The foregoing features may be more fully understood from the following description of the drawings. The drawings aid in explaining and understanding the disclosed technology. Since it is often impractical or impossible to illustrate and describe every possible embodiment, the provided figures depict one or more illustrative embodiments. Accordingly, the figures are not intended to limit the scope of the broad concepts, systems and techniques described herein. Like numbers in the figures denote like elements.

FIG. 1 is a block diagram showing an illustrative tunneling magnetoresistance (TMR) element having layers;

FIG. 2A is a graph of resistance versus field angle for a TMR element;

FIG. 2B is a graph of conductance versus field angle for a TMR element;

FIG. 3 is a circuit diagram of an example of a current-driven bridge having TMR elements;

FIG. 4 is a graph of normalized bridge output versus field angle for the current-driven bridge of FIG. 3;

FIG. 5 is a circuit diagram of an example of a voltage-driven bridge having TMR elements;

FIG. 6 is a graph of normalized bridge output versus field angles for the voltage-driven bridge of FIG. 5;

FIG. 7A is a graph of angle error versus pillar count for the current-driven bridge of FIG. 5;

FIG. 7B is a graph of angle error versus pillar count for the voltage-driven bridge of FIG. 3;

FIG. 8 is a circuit diagram of an example of a conductance bridge having TMR elements;

FIG. 9 is a graph of normalized bridge output versus field angles for the conductance bridge of FIG. 8;

FIG. 10 is a circuit diagram of another example of a conductance bridge;

FIG. 11 is a graph of angle error versus angle for a voltage-driven bridge and a conductance bridge;

FIG. 12A is a graph of normalized transfer function versus field for a single TMR element; and

FIG. 12B is a graph of normalized sensitivity versus field for a single TMR element.

DETAIL DESCRIPTION

Described herein are techniques to reduce voltage nonlinearity in a bridge having one or more tunneling magnetoresistance (TMR) elements.

Referring to FIG. 1, an illustrative TMR element can have a stack 150 of layers 158, 106-110, 156, 154, 152, 126 disposed upon a surface of a substrate 102. FIG. 1 is indicative of one pillar 160 of a multi-pillar TMR element. For example, a TMR element may be comprised of two or more pillars 160 that are identical and formed from stack 150.

It will be understood that a driving current running through the TMR element 150 runs through all of the layers of the stack, running between seed and cap layers 158 and 126 (i.e. perpendicular to a surface of the substrate 102) and the TMR element 150 is sometimes referred to as a current perpendicular-to-plane (CPP) magnetoresistance element. The TMR element 150 can have a maximum response axis that is parallel to the surface of the substrate and that is in the direction 128 perpendicular to zero field magnetic directions of the free layer 156, and also parallel to the bias field generated by the reference layers, most notably in the pinned layer 152.

The TMR element 150 is pinned (i.e. it has a pinning layer 106). A synthetic antiferromagnet (SAF) is formed from layers 108, 110 and 156. The aim of the SAF is to pin layer 156. The layer 156 is referred to as a reference layer. The single layer pinned layer 108 is magnetically connected to the pinning layer 106. The layers 106, 108 are collectively referred to as bias layers. With zero external magnetic field, the reference layer 156 takes on a magnetic alignment parallel to the bias layers 106, 108, with direction (ferromagnetic or antiferromagnetic coupling) determined by thickness and material of the spacer layer 110. In some embodiments, the single layer pinned layer 108 is replaced by another SAF structure.

As described above, in general, the TMR element 150 has a maximum response axis (maximum response to external fields) aligned with the arrow 128 (i.e. perpendicular to bias directions experienced by the free layer 156, and parallel to magnetic fields of the reference layers, notably pinned layers 152). Also, in general, it is rotations of the magnetic direction of the free layer 156 caused by external magnetic fields that result in changes of resistance of the TMR element 150.

Referring to FIG. 2A, a resistance of a tunneling magnetoresistance (TMR) element is not linearly dependent to a cosine of an angle between a free layer and a reference layer of the TMR element. For example, a cosine curve 202 represents a cosine of an angle between a free layer (e.g., free layer 156 (FIG. 1)) and a reference layer (e.g., the reference layer 152 (FIG. 1)) of the TMR element and a resistance curve 204 represents a resistance of the TMR.

Referring to FIG. 2B, unlike resistance, a conductance of a TMR element is linearly dependent to a cosine of an angle between a free layer and a reference layer of the TMR element. For example, a conductance curve 206 represents a conductance of the TMR element and is similar to a cosine curve 208 representing the cosine of an angle between a free layer (e.g., free layer 156 (FIG. 1)) and a reference layer (e.g., the reference layer 152 (FIG. 1)) of the TMR element.

Referring to FIG. 3, an example of a current-driven magnetometer bridge with TMR elements is a current-driven bridge 300. The current-driven bridge 300 includes TMR elements (e.g., a TMR element 308 a, a TMR element 308 b, a TMR element 308 c and a TMR element 308 d).

Each TMR element has a reference direction (e.g., similar to direction 128 (FIG. 1). For example, the TMR element 308 a has a reference direction 312 a, the TMR element 308 b has a reference direction 312 b, the TMR element 308 c has a reference direction 312 c and the TMR element 308 d has a reference direction 312 d. The reference direction 312 a is substantially in the same direction as reference direction 312 c but opposite (i.e. by 180°) from reference direction 312 b and reference direction 312 d.

The TMR element 308 a and TMR element 308 b are connected to a current source 304. In one example, the current source 304 supplies 0.5 mA. The TMR elements 312 c and TMR element 312 d are connected to ground 306.

The current-driven bridge 300 includes a node 332 between the TMR elements 308 a, 308 d and a node 334 between the TMR elements 308 b, 308 c. The output of the current-driven bridge 300 is the differential voltage across the nodes 332, 334.

Referring to FIG. 4, a cosine curve 402 represents a cosine of an angle between a free layer (e.g., free layer 156 (FIG. 1)) and a reference layer (e.g., the reference layer 152 (FIG. 1)) of the TMR element and an output curve 404 represents a normalized bridge output for the current-driven bridge 300. The output curve 404 does not match the cosine curve 402.

Referring to FIG. 5, an example of a voltage-driven bridge with TMR elements is a volt-driven bridge 500. The volt-driven bridge 500 includes TMR elements (e.g., a TMR element 508 a, a TMR element 508 b, a TMR element 508 c and a TMR element 508 d).

Each TMR element has a reference direction. For example, the TMR element 508 a has a reference direction 512 a, the TMR element 508 b has a reference direction 512 b, the TMR element 508 c has a reference direction 512 c and the TMR element 508 d has a reference direction 512 d. The reference direction 512 a is substantially in the same direction as the reference direction 512 c but opposite (i.e. by 180°) from the reference direction 512 b and the reference direction 512 d.

The TMR element 508 a and TMR element 308 b are connected to a voltage source 504. In one example, the voltage source 504 supplies 1.0V. The TMR elements 512 c and TMR element 512 d are connected to ground 506.

The voltage-driven bridge 500 includes a node 532 between the TMR elements 508 a, 508 d and a node 534 between the TMR elements 508 b, 508 c. The output of the voltage-driven bridge 500 is the differential current across the nodes 532, 534.

Referring to FIG. 6, a cosine curve 602 represents a cosine of an angle between a free layer (e.g., free layer 156 (FIG. 1)) and a reference layer (e.g., the reference layer 152 (FIG. 1)) of the TMR element and an output curve 604 represents a normalized bridge output for the voltage-driven bridge 500. Unlike the output curve 404 for the current-drive bridge 300, the output curve 604 for the voltage-driven bridge 500 is similar to the cosine curve 602.

As indicated in FIGS. 2A, 2B, 4 and 6, a voltage-driven bridge with TMR elements is more desirable than a current-driven bridge with TMR elements in reducing voltage non-linearity in a bridge.

Referring to FIGS. 7A and 7B, an error curve 702 represents angle error with respect to pillar count for a current-driven bridge and an error curve 704 represents angle error with respect to pillar count for a voltage-driven bridge. The curves 702, 704 indicate that increasing pillar count significantly reduces angle error in a voltage-driven bridge. For example, a pillar count of a 100 pillars can reduce the angle error to below 0.1°, a pillar count of more than a 1,000 pillars can reduce the angle error to below 0.01° and a pillar count of more than a 10,000 pillars can reduce the angle error to below 0.001° for an ideal TMR, i.e. of which free layer follows exactly the external field and the reference layer is completely insensitive to the external field. However increasing pillar count increases angle error in a current-driven bridge to a saturation point just under 8° but decreasing the pillar count improves the angle error in a current-driven bridge to about 2° or so for an ideal TMR.

Referring to FIG. 8, a conductance bridge configuration 800 includes TMR elements (e.g., a TMR MR element 808 a, a TMR element 808 b, a TMR element 808 c and a TMR element 808 d). The conductance bridge configuration 800 also includes an amplifier 816, an amplifier 822, a transistor 832, a transistor 834, a transistor 836 and a transistor 838.

The TMR element 808 a, 808 b are connected to a voltage source V_(reg) and the TMR elements 808 c, 808 d are connected to ground. A collector 832 c of the transistor 832 is connected to a collector 836 c of the transistor 836 and a collector 834 c of the transistor 834 is connected to a collector 838 c of the transistor 838.

Each TMR element 808 a-808 d has a reference direction. For example, the TMR element 808 a has a reference direction 812 a, the TMR element 808 b has a reference direction 812 b, the TMR element 808 c has a reference direction 812 c and the TMR element 808 d has a reference direction 812 d. The reference direction 812 a is substantially in the same direction as reference direction 812 c, but substantially in the opposite direction to reference directions 812 b, 812 d.

To hold constant the voltages across each TMR element 808 a, 808 b, 808 c, 808 d via a voltage feedback loop, the transistors 832, 834, 836, 838 may be used as a voltage follower. In one example, the transistors 832, 834 are PNP bipolar junction transistors and the transistors 836, 838 are NPN bipolar junction transistors. In another example, the transistors 832, 834 are p-type metal oxide semiconductor field-effect transistors and the transistors 836, 838 are n-type metal oxide semiconductor field-effect transistors. In other examples, the transistors 832, 834 are any combination of PNP bipolar junction transistors and p-type metal oxide semiconductor field-effect transistors and the transistors 836, 838 are any combination of NPN bipolar junction transistors and n-type metal oxide semiconductor field-effect transistors.

An output of the amplifier 816 is connected to a base 832 b of the transistor 832 and is connected to a base 834 b of the transistor 834. A first input of the amplifier 816 is connected to the TMR element 808 a and is connected to an emitter 832 a of the transistor 832. A second input of the amplifier 816 is connected to the TMR element 808 b and is connected to an emitter 834 a of the transistor 834. A third input of the amplifier 818 is connected to a current source 842 providing a current, I_(ref) and connected to a resistor 852 having a resistance R_(ref). The resistor 852 is connected to the voltage source V_(reg).

In one example, the resistor 852 and the current source 842 provide a reference voltage, V_(ref) to the amplifier 816. In one particular example, the reference voltage V_(ref) provided by the current source 842 and the resistor 852 is adjustable based on variations of one or both of the TMR elements 808 a, 808 b. For example, the current source 842 and/or the resistor 852 may be adjusted over temperature to avoid clipping of the output signal post amplification.

An output of the amplifier 822 is connected to a base 836 b of the transistor 836 and is connected to a base 838 b of the transistor 838. A first input of the amplifier 822 is connected to the TMR element 808 d and is connected to an emitter 836 a of the transistor 836. A second input of the amplifier 828 is connected to the TMR element 808 c and is connected to an emitter 838 a of the transistor 838. A third input of the amplifier 838 is connected to a current source 844 providing a current, I_(ref) and connected to a resistor 854 having a resistance R_(ref). The resistor 854 is connected to ground.

In one example, the resistor 854 and the current source 844 provide a reference voltage, V_(ref) to the amplifier 822. In one particular example, the reference voltage V_(ref) provided by the current source 844 and the resistor 854 is adjustable based on variations of one or both of the TMR elements 808 c, 808 d. For example, the current source 842 and/or the resistor 852 may be adjusted over temperature to avoid clipping of the output signal post amplification.

The conductance bridge configuration 800 includes a node 852 between the transistors 832, 836 and a node 854 between the transistors 834, 838. The output of the conductance bridge configuration 800 is the differential current across the nodes 852, 854.

While FIG. 8 depicts two amplifiers 816, 822, two additional amplifiers may be added so that each amplifier is connected to a respective one TMR element and a respective one transistor.

Referring to FIG. 9, a cosine curve 902 represents a cosine of an angle between a free layer (e.g., free layer 156 (FIG. 1)) and a reference layer (e.g., the reference layer 152 (FIG. 1)) of the TMR element and an output curve 904 represents a normalized bridge output for the conductance bridge 800. The output curve 904 for the conductance bridge is similar to the cosine curve 902.

Referring to FIG. 10, another example of a conductance bridge configuration is a conductance bridge configuration 1000. The conductance bridge configuration 1000 includes an amplifier 1012, an amplifier 1014, a resistor, R1 and a resistor R2.

The conductance bridge 1000 also includes a bridge 1002 that includes a TMR element 1022 a that has a resistance Rn1, a TMR element 1022 b having a resistance Rp1, a TMR element 1022 c having a resistance Rn2 and a TMR element 1022 c having a resistance Rp2. The TMR element 1022 a is connected to the TMR element 1022 c at a node 1024 that is connected to ground and to the TMR element 1022 b. The TMR element 1022 d is connected to the TMR element 1022 b at a node 1026 that is connected to V_(BIAS) and to the TMR element 1022 c. The bridge 1002 is biased with a constant voltage between the nodes 1024, 1026, which is a voltage V_(BIAS).

An output of the amplifier 1012 is connected to the resistor R1 at a node 1032 and an output of amplifier 1014 is connected to the resistor R2 at a node 1034. A first input 1012 a of the amplifier 1012 is connected to a first input 1014 a of the amplifier 1014 and each input 1012 a, 1014 a receives a voltage V_(BIAS)/2.

A node 1016 is connected to a resistor R1, a second input 1012 b of the amplifier 1012, the resistor Rp1 and the resistor Rn1. The current at the node 1016 is equal to: V _(BIAS)/(2Rp1)−V _(BIAS)/(2Rn1).

A node 1018 is connected to a resistor R2, a second input 1014 b of the amplifier 1014, the resistor Rp2 and the resistor Rn2. The current at the node 1018 is equal to: V _(BIAS)/(2Rp2)−V _(BIAS)/(2Rn2).

A voltage at the node 1032 is a voltage V01. Assuming the current into the amplifier 1012 is negligible V01 is equal to: [V _(BIAS)/(2Rp1)−V _(BIAS)/(2Rn1)]R1.

A voltage at the node 1034 is a voltage V02. Assuming the current into the amplifier 1014 is negligible V01 is equal to: [V _(BIAS)/(2Rp2)−V _(BIAS)/(2Rn2)]R2.

If the output of the bridge configuration 1000 is VOUT and VOUT=V01−V02, then VOUT is equal to: [V _(BIAS)/(2Rp1)−V _(BIAS)/(2Rn1)]R1−[V _(BIAS)/(2Rp2)−V _(BIAS)/(2Rn2)]R2.

In one example, V_(BIAS) is adjusted with temperature such that variations caused by temperature to the TMR elements 1022 a, 1022 b, 1022 c, 1022 d are minimized with respect to VOUT.

Referring to FIG. 11, an error curve 1102 for voltage-driven bridges using an ideal TMR indicates that the error is relatively small between 0.06° and −0.06° for all angles and an error curve 1104 for a conductance bridge using an ideal TMR has an error about 0.0°. FIG. 11 depicts error values for the voltage-driven bridge and for the conductance bridge where each TMR element has thirty pillars for each TMR element and the conductance bridge uses 0.5V per TMR element. In another example not shown in FIG. 11, a voltage-driven bridge having one pillar for each TMR element has an angle error between 0.5° and −0.5°.

Referring to FIGS. 12A and 12B, while the disclosure thus far indicates that conductance in a TMR element can be reduced to voltage linearity errors in an angle sensor, conductance in a TMR may be used to reduce voltage linearity errors in linear sensors. For example, a normalized resistance curve 1202 is different than a normalized conductance curve 1204. In particular, a resistance sensitivity curve 1222 is less symmetrical than a conductance sensitivity curve 1224 even for current or voltage driven bridges.

The processes and techniques described herein are not limited to the specific examples described herein.

Elements of different embodiments described herein may be combined to form other embodiments not specifically set forth above. Various elements, which are described in the context of a single embodiment, may also be provided separately or in any suitable subcombination. Other embodiments not specifically described herein are also within the scope of the following claims. 

What is claimed is:
 1. A magnetic field sensor, comprising: a plurality of tunneling magnetoresistance (TMR) elements comprising a first TMR element, a second TMR element, a third TMR element and a fourth TMR element, wherein the first and second TMR elements are connected to a voltage source, wherein the third and fourth TMR elements are connected to ground, wherein each TMR element has a pillar count of more than one pillar, wherein the pillar count is selected to reduce the angle error below 1.0°.
 2. The magnetic field sensor of claim 1, wherein the pillar count is selected to reduce the angle error below 0.1°, wherein the angle error is caused by voltage nonlinearity.
 3. The magnetic field sensor of claim 1, wherein the pillar count is selected to reduce the angle error below 0.01°, wherein the angle error is caused by voltage nonlinearity.
 4. The magnetic field sensor of claim 1, wherein the pillar count is selected to reduce the angle error below 0.001°, wherein the angle error is caused by voltage nonlinearity.
 5. The magnetic field sensor of claim 1, wherein each TMR element has a voltage drop between 0V and 1.5V.
 6. The magnetic field sensor of claim 1, wherein each TMR element has a voltage drop of 0.5V.
 7. The magnetic field sensor of claim 1, wherein the magnetic field sensor is an angle sensor.
 8. The magnetic field sensor of claim 1, wherein the magnetic field sensor is a linear sensor.
 9. The magnetic field sensor of claim 1, wherein the magnetic field sensor is a rotary sensor.
 10. The magnetic field sensor of claim 1, wherein the voltage source provides a voltage between 0V and 1.5V.
 11. The magnetic field sensor of claim 1, wherein the voltage source provides a voltage between 1.0V.
 12. The magnetic field sensor of claim 1, wherein a reference direction of the first TMR element is opposite a reference direction of the second TMR element.
 13. The magnetic field sensor of claim 12, wherein the reference direction of the first TMR element is opposite a reference direction of the third TMR element.
 14. The magnetic field sensor of claim 13, wherein the reference direction of the first TMR element is equal to a reference direction of the fourth TMR element.
 15. The magnetic field sensor of claim 1, wherein a reference direction of the first, second, third and fourth TMR elements are in the same direction.
 16. The magnetic field sensor of claim 1, wherein each TMR element comprises a free layer that changes with an applied field and a reference layer that is fixed. 